Head-up displays have long been used in airplanes to aid the pilots. The head-up display reflects information the pilot needs off of a partial mirror or the windshield (combiner) and projects that information at a distance in front of the plane so that the pilot does not have to look into the cockpit to see the information.
More recently, head-up displays have been used in automotive applications in which the image is projected up and reflected off of the vehicle front windshield, to appear at a distance in front of the driver in combination with scenery forward of the vehicle. With a head-up display in a vehicle, the driver does not have to adjust his/her eyes from the road to read information, such as vehicle speed, which is normally displayed in the vehicle instrument panel.
When the windshield is used as a combiner, a bit of complexity is added to the system. Vehicle windshields are imperfect optical elements in that they are aspherical and that they ave two reflecting surfaces, tending to cause ghost images, resulting in a projected display appearing as a double image. Corrections for these imperfections add cost and complexity to the system. For example, in some systems, an aspheric mirror is used to compensate for the aspheric curvature of the windshield so that a clear image is produced. However, for every vehicle with a different windshield design, a new aspheric mirror design must be developed, adding expense when expanding the head-up display system to new vehicle models.
When a combiner separate from the windshield is used, the complexities caused by the vehicle windshield are eliminated. However, previous head-up displays including a combiner separate from the windshield have so far not been widely used by vehicle manufacturers for bulk, style and other reasons. One of these other reasons is the expense required with systems that include numerous parts.